The present invention relates to the field of wireless telecommunications networks, in particular wireless local access networks (WLANs) in compliance with the IEEE 802.11 family of standards.
Such networks are also known as WiFi networks. They are used in numerous applications for networking stations (e.g. computers, personal digital assistants, and peripherals).
In document “IEEE 802.11a-1999, IEEE 802.11b-1999, IEEE 802.11 d-2001, Part 11: wireless LAN medium access control (MAC) and physical layer (PHY) specifications” the 802.11 standard defines a method of regulating traffic in a wireless network. That method makes use of a congestion window (CW) system for regulating the traffic. According to that standard, in order to determine the instant at which a data frame is to be transmitted, a station draws a random number in the range 0 to CW-1, where the value CW is an integer lying in the range CWmin to CWmax as specified by the 802.11 standard.
The number that has been drawn is used for a countdown prior to transmitting the frame, the countdown being deferred if the station finds that some other station is transmitting. Unfortunately, that congestion window system leads to a large number of collisions in the wireless network, and from the point of view of the user that leads to a significant loss of bandwidth.
A mechanism known as the “tournament method” can be used by the various stations for regulating the transmission of frames and for limiting the collision rate. That tournament method is described in the document by Z. Abichar and M. Chang entitled “CONTI: constant time contention resolution for WLAN access”, IFIP Networking 2005.
The tournament method consists in organizing a kind of tournament between stations having at least one frame to transmit. A tournament is made up of a certain number of selection rounds, each of which is executed during a time interval of predefined duration that enables each station to have the time to listen for transmission by any other station. This duration is typically equal to the duration of a “SlotTime” that, according to the IEEE 802.11 standard, is defined as the elementary time interval in the procedure for resolving contention by the congestion window. At the beginning of the tournament, it can happen that all of the stations having a frame to transmit are authorized to transmit that frame. On each selection round, one or more stations can be eliminated from the list of stations that are authorized to transmit depending on whether during that selection round a station has had allocated thereto a transmission authorization or a transmission prohibition. At the end of the tournament, only those stations that have not been eliminated are authorized to transmit. If a plurality of stations are still in the running at the end of the tournament, then they transmit simultaneously, thereby giving rise to a collision and thus to disturbed reception, with it being impossible to receive the transmitted data frames correctly. Those stations must therefore participate in the following tournament in order to make another attempt at transmitting those frames.
The Hiperlan protocol is one of the first protocols to incarnate the tournament function.
For further information, the person skilled in the art may refer to the document by Philippe Jacquet, Pascale Minet, Paul Mühlethaler, and Nicolas Rivierre “Priority and collision detection with active signaling—the channel access mechanism of HIPERLAN”, Wireless Personal Communications 4: 11-26, 1996.
Documents FR 2 893 206 and WO 2009/095628 explain respectively how to improve considerably the effectiveness of tournaments by making the probabilities of the draw depend on the history of the tournament, and how to guarantee bounded access times to stations in order to enable synchronous streams to be transmitted over wireless networks with good quality of service.
In prior art tournaments, the probability of a negative outcome, i.e. of transmission with collision when at least two stations consider that they have won the tournament, diminishes asymptotically with ½kmax, where kmax is the number of selection rounds. In order to guarantee good performance, the number of rounds, and thus the duration of the tournament, must be greater than a certain threshold. Thus, the Applicant has found that networks implementing a tournament method can suffer from a poor utilization rate of their bandwidth since, in a six-round tournament by way of example, the length of time taken to execute the contention resolution protocol can represent about 25% of the time needed for transmitting a data frame over an IEEE 802.11n network.
In addition, in a network in which certain stations perform a tournament method, while other stations perform a congestion window procedure, of the kind mentioned above, then collisions can lead to a tournament failing.